The present invention relates to composite ion exchange membranes, and in particular, composite ion exchange membranes for use in solid polymer electrolyte fuel cells.
Ion exchange membranes are used in a variety of applications. For example, ion exchange membranes are components of electrochemical cells such as solid polymer electrolyte fuel cells, chlor-alkali electrolysis cells, and batteries. Ion exchange membranes are also employed in diffusion dialysis, electrodialysis, pervaporation, and vapor permeation applications. Anion, cation, and amphoteric ion exchange membranes are known.
Ion exchange membranes may comprise dense polymer films. For example, Nafione(copyright) membranes are commercially available dense film perfluorosulfonic acid ion exchange membranes suitable for use in solid polymer electrolyte fuel cells and chlor-alkali electrolysis cells. As another example, commonly assigned U.S. Pat. No. 5,422,411, incorporated herein by reference in its entirety, describes dense ion exchange membranes comprising polymeric compositions comprising substituted xcex1,xcex2,xcex2-trifluorostyrene monomers. Current dense film ion exchange membranes suffer certain practical limitations for use in electrochemical cells such as fuel cells, such as cost and thickness, for example.
For ease of handling, for example, in the preparation of membrane electrode assemblies (xe2x80x9cMEAxe2x80x9d) for use in fuel cells, the mechanical strength of the membrane in the dry state and hydrated state is important. In electrochemical applications, such as electrolytic cells and fuel cells, the dimensional stability of the membrane during operation is also important. Further, to improve performance, it is generally desirable to reduce membrane thickness and to decrease the equivalent weight of the membrane electrolyte, both of which tend to decrease both the mechanical strength and the dimensional stability in the hydrated state.
One approach for improving mechanical strength and dimensional stability relative to dense film ion exchange membranes is through the use of a porous reinforcing support material. For example, an unsupported membrane can be preformed and then laminated to the reinforcing support, or a dense film may be formed directly on a surface of the reinforcing support. The reinforcing support is typically selected so that it imparts some mechanical strength and dimensional stability relative to the dense film ion exchange membrane. Composite membranes (discussed below) have also been laminated with reinforcing supports to form reinforced membranes.
Laminating or otherwise combining a reinforcing support with a dense film membrane or a composite membrane, while increasing mechanical strength and dimensional stability, is not totally beneficial. One reason is that the reinforcing support tends to defeat the purpose of a thin membrane by increasing the overall thickness. Another reason, which also leads to reduced ionic conductivity, is due to the xe2x80x9cshadowingxe2x80x9d effect of the reinforcing support. The shortest path for an ion through a membrane is a perpendicular path from one surface to the other surface. Reinforcing supports are typically made from materials that are not ion-conductive. Those parts of the reinforced ion exchange membrane where an ion cannot travel perpendicularly across the membrane, but must take a circuitous route around the reinforcing support, are xe2x80x9cshadowedxe2x80x9d areas. The presence of shadowed areas in the reinforced membrane reduces the effective area of the membrane that actively conducts ions, thereby decreasing the effective ionic conductivity of the membrane.
Another approach for improving mechanical strength and dimensional stability in ion exchange membranes is to impregnate an ion-conductive material into a porous substrate material to form a composite membrane. Such composite ion exchange membranes prepared by impregnating commercially-available microporous polytetrafluoroethylene (ePTFE) film (Gore-Tex(copyright); W.L. Gore and Associates, Inc., Elkton, Md.) with Nafion(copyright), have been described in the Journal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major goal in the study was to develop a composite membrane with the desirable features of Nafion(copyright), but which could be produced at a low cost. Similarly, U.S. Pat. Nos. 5,547,551, 5,599,614 and 5,635,041 describe composite membranes comprising microporous expanded PTFE substrates impregnated with Nafion(copyright). Gore-Select(copyright) membranes (W.L. Gore and Associates, Inc., Elkton, Md.) are composite membranes comprising a microporous expanded PTFE membrane having an ion exchange material impregnated therein.
Composite membranes incorporating other porous substrate materials, such as polyolefins and poly(vinylidene fluoride) and other ion exchange materials, have also been described. For example, commonly assigned U.S. Pat. No. 5,985,942, incorporated herein by reference in its entirety, describes composite membranes comprising a porous substrate and, inter alia, ion exchange materials comprising substituted xcex1,xcex2,xcex2-trifluorostyrene polymers and copolymers.
Composite ion exchange membranes suitable for use in fuel cells, in addition to having suitable mechanical strength and dimensional stability, should also have suitable ionic conductivity and be substantially impermeable to gas reactants. To achieve these aims, current composite ion exchange membranes, such as the Gore-Select(copyright) membranes, are relatively thin and the microporous substrate is impregnated throughout with an ion exchange material. These composite ion exchange membranes are also typically uniform and integral, meaning a continuous impregnation of the microporous membrane such that no pin holes or other discontinuities exist within the composite structure.
While current composite ion exchange membranes developed for use in fuel cells have achieved a measure of success, there are still areas for additional improvement. First, as noted above, the microporous substrate is filled with ion exchange material. Generally speaking, the ion exchange material is the most expensive component of the composite. Thus, essentially the maximum cost of ion exchange material is incurred for a given thickness of microporous substrate in current composite ion exchange membranes for use in fuel cells. Second, current methods for producing such composite ion exchange membranes typically involve multiple coating steps to fully impregnate the substrate with ion exchange material. Alternatively, or in addition, such methods comprise steps for facilitating impregnation, such as ultrasonication, or adding surfactants to the impregnation solution. These steps increase the time, complexity, and cost of producing composite ion exchange membranes. This is particularly the case where surfactants are added to the impregnation solution, which generally necessitates an additional processing step to remove the surfactant before using the composite membrane in a fuel cell.
It is desirable to have a composite ion exchange membrane suitable for use in fuel cells that is less expensive and easier to produce than current composite ion exchange membranes and that provides comparable fuel cell performance.
A composite membrane and methods for making the composite membrane are provided. In one embodiment, the present composite membrane is an asymmetric composite membrane for use in a fuel cell membrane electrode assembly, and the composite membrane comprises:
(a) a porous polymeric substrate;
(b) an impregnant comprising a cation exchange material, the impregnant partially filling the substrate such that the substrate comprises a first region having pores substantially filled with the impregnant, and a second substantially porous region; and
(c) a dense surface layer comprising the cation exchange material, the dense layer contiguous with the first region of the substrate,
wherein the substrate has greater than 10% residual porosity, and the composite membrane is substantially gas impermeable and has a substantially porous major surface.
In another embodiment, the present composite membrane comprises:
(a) a porous polymeric substrate; and
(b) an impregnant comprising at least one cation exchange material, the impregnant partially filling the substrate such that the substrate comprises a first region having pores substantially filled with the impregnant, and a second substantially porous region,
wherein the substrate has greater than 10% residual porosity, and the composite membrane is substantially gas impermeable and has at least one substantially porous major surface.
An embodiment of the method of making the present composite membrane comprises:
(a) impregnating a porous polymeric substrate by contacting a first impregnant solution with one major surface of the substrate, the first solution comprising at least one polymer and a solvent; and
(b) removing the solvent from the first solution by evaporation.
In another embodiment, the present composite membrane comprises:
(a) two porous polymeric substrates; and
(b) an impregnant comprising at least one cation exchange material, the impregnant partially filling each of the substrates such that each substrate comprises a first region having pores substantially filled with the impregnant, and a second substantially porous region, each of the first regions in contact with the dense layer,
wherein the composite membrane has greater than 10% residual porosity, is substantially gas impermeable, and has two substantially porous major surfaces.
Another embodiment of the method of making the present composite membrane comprises:
(a) impregnating a first porous polymeric substrate to form a first layer by contacting a first impregnant solution with one major surface of the first substrate, the first impregnant solution comprising at least one polymer and a solvent;
(b) removing the solvent from the first solution by evaporation;
(c) impregnating a second porous polymeric substrate to form a second layer by contacting a second impregnant solution with one major surface of the second substrate, the second impregnant solution comprising at least one polymer and a solvent;
(d) removing the solvent from the first solution by evaporation; and
(e) laminating the first and second layers together.
The impregnant may comprise a polymer containing precursor substituents that can be converted into ion exchange substituents. Suitable percursor substituents include xe2x80x94SO2X, xe2x80x94SO2OR, xe2x80x94SR, xe2x80x94NRRxe2x80x2, and xe2x80x94PO(OR)2 (where X=Br, Cl, F; and R, Rxe2x80x2 can be alkyl or aryl). In such cases, the method for making the composite membrane may further comprise converting the percursor substituents to ion exchange substituents.
In making the present composite membrane, an embodiment of the present method comprises impregnating the porous substrate(s) with an impregnant comprising a polymer and then introducing ion exchange substituents into the polymer post-impregnation.
The present composite membrane may further comprise an electrochemically inert, hygroscopic material. The hygroscopic material may be present in the substrate material, the impregnant, the dense layer (if present), or any combination thereof. The impregnant may also further comprise compatible mixtures of non-ionic polymers, if desired.
In the present composite membrane the substrate may have greater than 15% residual porosity. For example, it may have between about 15% and about 20% residual porosity.
Membrane electrode assemblies and fuel cells comprising the present composite membrane are also provided.